Some hybrid vehicles use a BAS (belt alternator starter) connected to the engine crankshaft for engine starting and regenerative braking A. BAS arrangement requires that the engine crankshaft must spin when the alternator-starter is either producing electricity from regenerative braking or providing electric propulsion. Spinning the engine during these times produces mechanical losses from engine friction and from engine pumping losses if valve deactivation is not provided, diverting some of the energy from being useful.
Strong hybrid architectures have been proposed to couple a motor/generator, either directly or indirectly, to the transmission input, transmission output, or an intermediate shaft within the transmission, such as a layshaft. However, this same connection of the motor/generator (i.e., the direct or indirect connection to the transmission input, transmission output, or intermediate shaft) is also used for starting the engine, which limits the favorable gear ratio between the motor-generator and the engine to a level that is well below that provided by starter ring and pinion gears. For instance, a hybrid architecture that has the motor-generator coupled directly to the transmission output shaft has only the reciprocal of the lowest transmission speed ratio, which might amount to a ratio of approximately 2:1, to multiply the torque from the motor-generator to start the engine, while a typical starter ring and pinion might have a ratio of 10:1 or more.